Okay, all of these galactic alignment in 2012 people have gotten me thinking. When pondering my response to one of them (why do I even bother?), something dawned on me, and I would like to know if I thought about it correctly or not.
If the poles are progressing over a 20+ thousand year cycle, doesn't this mean that the Sun's position in the zodiac also changes? In other words, would the Sun be in the constellation Sagitarius during the month of March some 7000 years from now?
We use leap years to keep our calendar aligned with the seasons, but wouldn't the stellar background drift over time as a result? If I'm correct in my thinking, then in about 13000 years, northern summer will be at perihelion in stead of the apahelion that it is currently. I suspect we'll want to call that "July" still, so that means the sun would be 180 degrees through the sky from where it is now. Is this right, or am I missing something?
ZikZak2008-01-22T11:57:05Z
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You've got it pretty much right.
The calendar is designed to maintain coordination with the tropical, or seasonal year, which is the time from one vernal equinox to the next. That's different from the anomalistic year, which is the time from one perihelion to the next. So yes, the perihelion drifts through the calendar. It's also different from the sidereal year (the time between two closest conjunctions of the Sun with any fixed star), so the signs of the zodiac also drift through the calendar.
If I understand your question, I don't think it would have any real effect because of the way the equinoxes and solstices are defined. Even though the Sun is moving through space, and thus the stars are changing position, the Earth will still be moving around the Sun normally. The equinoxes and solstices are defined by the apparent position of the Sun from the vantage point of the Earth. The Sun may very well be in a different constellation 7000 years from now (an astronomer could answer that better), but someday in late March of the year 9000 the Sun will still appear to be directly over the equator, which will define the start of Spring in the northern hemisphere.
The precession of the equinoxes has been known since antiquity, but is apparently a mystery to Yahoo Answers users.
You're right. The seasons are slowly shifting through the calendar, and will continue to do so. The visibility of stars changes too: this is why southern constellations like Centaurus and Lupus were identified by the ancients. They could see them, but due to the precession of the equinoxes, we cannot see them from northern locations. Indeed, this is how the precession of the equinoxes was discovered in the first place.
The Earth's perihelion and aphelion change too, BTW.
It doesn't make the slightest difference to 2012 kookery, however. Whether you consider the precession of the equinoxes or not, it's still nonsense.
Your logic is correct about the sun shifting in the zodiac. The reason we have the weird pattern of leap years is to keep the calendar in sync with the seasons, so the sun's path will cross the equator on about the same days (late March and late September) from year to year.
The "precession of the equinoxes" -- i.e., the wobbling of the earth's axis like a top over a 26000 year period -- causes the intersections of the ecliptic with the equator to shift over time. If the sun lies in some zodiacal constellation now on a given day of the year, it will lie in the opposite zodiacal constellation on the same day in 13000 years.
The situation regarding perihelion is a bit more complicated, however. The earth's orbit is not perfectly stable because of perturbations by other objects in the solar system (particularly Jupiter). You would think (actually I recently thought) that perihelion will shift by 6 months in 13000 years; but the axis of the earth's orbit is also rotating! As a result of these two effects in combination, the perihelion shifts by 6 months in only 10,500 years.
-- edit Here's the basic data for these effects. We define years of different lengths, and here are three key ones: T1 = 365.242190 days = tropical year (equinox to equinox) T2 = 365.256363 days = sidereal year (fixed star to fixed star) T3 = 365.259635 days = anomalistic year (perihelion to perihelion)
The length of the cycle for precession of the equinoxes is T2/(T2-T1) tropical years = 25771 tropical years (Over this length of time, the number of tropical years will exceed the number of sidereal years by one.)
Similarly, the length of the cycle for perihelion is T3/(T3-T1) years = 20938 years So in half this time (10469 years), the times of perihelion and aphelion in the calendar will be the reverse of what they are today. That will make northern-hemisphere seasons a bit more extreme, and southern-hemisphere seasons a bit less extreme.
note: These numbers (25771 and 20938 years) are given to a little too much precision. I don't think they're correct to the nearest year, but they're very close. Wikipedia says the precession cycle is 25765 years long. A lot of sources list the perihelion cycle as roughly 21000 years.
This change in the axis of the earth's orbit relative to the fixed stars is known as precession in perihelion. This happens to other planets also, and the precession of the perihelion of Mercury is particularly famous. Nineteenth-century celestial mechanics observed a small discrepancy in Mercury's perihelion precession that they were unable to explain. It remained a mystery until Einstein showed that Newton's theory of gravitation was incorrect; the discrepancy was fully explained by Einstein's theory of general relativity. Of course, the discrepancy would never have been known without the incredible precision provided by Newton's theory. Newton said that he saw farther than others because he stood on the shoulders of giants, and the same is true of Einstein.
one more thing: It's important to realize that cause of the seasons is the tilt of the earth's axis; the varying distance from the sun is a much smaller effect. Therefore, in 10500 years, July will still be a hot summer month in the northern hemisphere because that hemisphere will still tilt towards the sun in July; that's because our calendar is a good approximation to the tropical year. The fact that perihelion will occur in July instead of January will make northern-hemisphere summers a little warmer than they are now, and northern-hemisphere winters a little colder (other things being equal).
Yes, you are correct. In 13,000 years the vernal equinox will be in Virgo, and Orion will be a summer constellation in the northern hemisphere. There will come a time when perihelion happens during northern summer, but it won't be then, as the position of perihelion also moves.